i read thru the manual for the CDA-9857, and as you mentioned, the 9856 is the same but without the remote (i could never understand the *need* for a remote in a car... just another piece to lose or break. maybe it's for people with extremely short arms?) :lol:
it looks like there is only one set of speaker outputs, and that you can toggle the internal amp on and off. so if you don't use an external amp, you leave the internal amp on and wire the speakers directly to the H/U. if you DO use an external amp, you turn off the internal amp and connect the speaker outs to the amp inputs, then connect the speakers to the amp outs. make sense?
the MBQ dash speakers you've selected are adequate, and they are what i've used. you might want to think about getting the next model up the line. don't get me wrong, the speakers are fine, but if you spent twice as much (around $80) i think you would be happier. i haven't tried the door speakers you mentioned, but they look fine. keep in mind that if you get an amp with crossovers built-in, you won't *need* to use component speakers. in my doors i used 6.5" MBQ DKE-116s (coax speakers).
there are many amps to choose from. the standard Boxster amps are 40w per channel, and the sound is decent and semi-loud, but i think that you'd want t a little more than the 40w per channel. i'd look for a 6 channel amp with 50 to 100w per channel. you may be able to get away with a 4 channel amp and connect the dash and rear speakers to the same amp outputs (2 channels) and the door speakers to the other 2 channels. check out www.woofersetc.com and look under amplifiers. there are MANY to choose from. i would also call a stereo installer, tell them what you want to do, and let them recommend an amp and how they would connect everything (4 channel vs 6 channel, etc., wiring speakers in parallel, etc.). then go buy the amp and do it yourself. ;) don't worry, they'll make money off some kid who doesn't know how to do it for himself. on the other hand, it might be worth it to you to pay the $100-$200 (or more) to have them do it. it really depends on how comfortable you are with DIY. of course, you could always try it yourself and if you screw it up bad enough, bring it to them. that's my motto. :lol:
when installing the amp, you'll need to find a suitable location for it. the Boxster amps are mounted against the trunk firewall in the front trunk. depending on the size of your amp, you could do that, or mount it on the floor of the front trunk, against the front wall of the trunk, or wherever it will fit. you'll want to hook it up to switched power so that the amp is only on when the stereo is on.
all this wiring stuff is complicated and very long to type, so when you get all your stuff ready to go, post here or PM me and i will give you a hand. ;)
i have heard that the PNP speaker kits from Harvey on eBay are very good, but they are too expensive for me. so i made my own, but my car is a '97. what does your rear storage shelf look like in an '04 (the one behind the seats). is it the hard plastic shelf? i'll try to find a picture to get a better idea. the basic idea to make your own is to buy some speakers that you like (i bought Pioneer TSA878 3.5" speakers http://www.woofersetc.com/index.cfm?fuseac...Product_ID=1014 and cut a piece of wood to mount the speakers and then just wired them up. they sound GREAT!! so ~$50 DIY vs $250 for a kit... hmmm. ;)